This invention relates to metal working, and more particularly to metal working in the field of aircraft manufacturing utilizing metal alloys capable of superplastic behavior.
The use of superplastic metal alloys as part of a process for forming metal structures has been known for some time. The first U.S. patent to disclose superplastic metal working was U.S. Pat. No. 3,340,101 to Fields et al for "Thermoforming of Metals" (1967), hereby incorporated by reference. This patent explains the limitations of other methods and suggests the extreme deformability that can be attained using superplasticity.
U.S. Pat. No. 3,895,436 to Summers et al for "Forming Metals" (1975), hereby incorporated by reference, discloses a process for forming a metallic vessel, the process including the steps of forming an inflatable envelope of a superplastic metallic alloy, heating the envelope to within the temperature range for superplasticity, and applying a differential pressure between the interior and exterior of the envelope such that the envelope expands like a balloon.
U.S. Pat. No. 3,896,648 to Schertenleib for "Blow Molding Process for Container of Superplastic Alloy", hereby incorporated by reference, discloses a method in which, within a metal mold, a smoothed hollow cylinder of superplastic alloy with a bottom is preheated and partially inflated by the application of a first internal pressure, and then blown out to its final dimensions by a second, higher internal pressure.
U.S. Pat. No. 4,045,986 to Laycock et al for "Forming Ductile Materials" (1977), hereby incorporated by reference, discloses a process by which a sheet of ductile metallic superplastic alloy material is first forced by a pressure differential into a female portion of a pre-form mold, and then forced by a reversed pressure differential into conformity with a male portion of the mold which has advanced to press against the opposite side of the sheet.
U.S. Pat. No. 4,354,369 to Hamilton for "Method for Superplastic Forming", hereby incorporated by reference, discloses a method for eliminating or minimizing cavitation and voids in superplasticly formed parts by applying pressure to both sides of the material either during or after forming the part.
None of the above prior art patents for superplastic metal working appear to provide any means for preventing thinning of the superplastic material as it is expanded and deformed. Yet, thinning can be a very serious problem when the dimensions of a desired part must greatly exceed the dimensions of the blank piece of material from which it is to be formed.
A number of U.S. patents disclose one or another variation on the idea of applying axial compressive loading to a tubular work piece, while also using a liquid (or flowable solid) to raise the pressure inside the work piece, so that bulging or similar shaping of the work piece is accomplished without thinning of the material in the regions that are caused to bulge. For example, U.S. Pat. No. 3,974,675 to Tominaga for "Molding Device", hereby incorporated by reference, discloses a system in which a work piece is subjected to axial compression as it is caused to bulge by an instantaneously generated high hydraulic pressure.